Some owners of a bicycle may wish to upgrade their bicycle by adding a power unit. Adding a power unit, including adding a drive sprocket to engage and drive the chain on the bicycle, is a convenient way to perform this upgrade. However, similar to the situation of the second rider on a bicycle built for two, a drive sprocket arranged to engage the chain between a pedal sprocket and the rear wheel sprocket of the bicycle will only engage a small portion of the chain, and will therefore only transfer a small amount of torque.